1. Field of the Invention
The present invention relates to a carrier for electrophotography used for developing latent electrostatic images in electrophotographic process, electrostatic recording, electrostatic printing, or the like. The present invention further relates to a developer, a developer container, an image forming method, an image forming apparatus, and a process cartridge in each of which the carrier for electrophotography is used.
2. Description of the Related Art
In image formation according to an electrophotographic process, a latent electrostatic image is formed on an image bearing member which comprises a photoconductive substance, and the like (an image bearing member may be hereinafter referred to as latent electrostatic image bearing member, electrophotographic photoconductor, or photoconductor); a charged toner is made to adhere on the latent electrostatic image to form a visible image (a toner image), and the visible image is transferred onto a recording medium such as paper and fixed to thereby output an image.
In recent years, the developments of technologies for copiers and printers based on electrophotography are rapidly turning from monochrome systems to full-color systems, and the market of full-color systems tends to expand. In color image formation according to a full-color electrophotographic process, typically, three color toners of yellow, magenta, and cyan, or four color toners having a black toner in addition to theses three color toners are superimposed to thereby reproduce all colors. Thus, in order to obtain a brilliantly-colored full-color image having excellence in color reproductivity, the amount of light scattering should be reduced by smoothing the surface of a toner image to some extent. For this reason, most of the images formed by conventional full-color copiers fall in the range of moderate to high glossiness of 10% to 50%.
Generally, for a method for fixing a toner image on a recording medium, a contact heat fixing method is frequently used, in which a toner image is fixed by heating a roller and/or a belt having a smooth surface, pressurizing and contacting a toner with the roller and/or the belt. The method has advantages in that high thermal efficiency is obtained, high-speed fixing is possible, and it is possible to provide with glossiness and transparency in color toners, at the same time, the method causes a so-called offset phenomenon in which a part of a toner image adheres on the surface of a fixing roller and transfers onto another image, because the surface of a heat-fixing member such as a fixing roller is made to contact with a molten toner under increased pressure, and then the fixing member is isolated from the toner.
Aiming at prevention of the offset phenomenon, a method has been employed in which a surface of a fixing roller is formed with a silicone rubber or a fluorine resin which excels in releasing property, and is further coated with a releasing oil such as a silicone oil on the surface of the fixing roller. According to the method, it is extremely effective in preventing offset of toner, however, the method needs a device for supplying a releasing oil, and the fixing unit inevitably needs to grow in size, and the method is unfit for down-sizing. For this reason, in monochrome toners, another method tends to be increasingly employed in which no releasing oil is coated on a fixing roller (oil-less) or only a small amount of releasing oil is used by controlling the molecular mass distribution of a binder resin not so as to internally rupture a molten toner to increase viscoelesticity of the molten toner and further using a releasing agent such as wax to be included in the toner.
Also, in color toners, as is the case with monochrome toners, there are tendencies of employing oil-less process for the purposes of downsizing of machine and simplification of toner material composition. However, as mentioned above, with color toners, the surface of a fixed image needs to be smoothed to increase color reproductivity and thus viscoelasticity of a molten toner must be reduced. Therefore, color toners may more easily cause offset phenomena than in monochrome toners which have less glossiness, and it is more difficult to employ an oil-less process in fixing units as well as to coat a small amount of a releasing oil on fixing rollers or the like. In addition, when a releasing agent is included in a toner, transferring property of the toner to recording media is reduced due to increased adherence of the toner. Further, there is a problem that the releasing agent in the toner contaminates frictional charge members such as carrier, resulting in degradation of durability of the toner due to reduced charge property of the toner.
On the other hand, with respect to carriers, there has been a method in which a hard and high-strength coating layer is formed on a carrier for the purposes of preventing toner component-filming on a surface of carrier, uniformly forming a surface of carrier, preventing oxidation of a surface of carrier, preventing reduction in moisture sensitivity, prolonging developer life, preventing a carrier from adhering on surfaces of photoconductors, protecting photoconductors from flaws and frictions which are caused by a carrier, controlling charge polarity, and adjusting the charged amount or the like. There have been carriers proposed so far, for example, a carrier which is coated with a specific resin material (see Japanese Patent Application Laid-Open (JP-A) No. 58-108548); a carrier in which various additives are added to a coating layer (see Japanese Patent Application Laid-Open (JP-A) Nos. 54-155048, 57-40267, 58-108549, 59-166968, and 06-202381, and Japanese Patent Application Publication (JP-B) Nos. 01-019584, and 03-000628); a carrier having additives adhered on the surface thereof (see Japanese Patent Application Laid-Open (JP-A) No. 05-273789); a carrier in which conductive particles each having a diameter greater than the thickness of a coating layer are included in the coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 09-160304), and the like. In addition, Japanese Patent Application Laid-Open No. 08-006307 proposes to use a carrier-coating material which comprises a primary component of a benzoguanamine-n-butyl alcohol-formaldehyde copolymer. Japanese Patent (JP-B) No. 2683624 proposes to use a cross-linking product of a melamine resin and acrylic resin as a carrier coating material.
However, with these related art approaches, there are still problems with carrier adhesion in half-tone image parts where a number of dots are formed in early stage of image formation, carrier adhesion in solid image parts with the lapse of running time, and durability of carrier. With respect to early-stage carrier adhesion, it is problematic because the carrier adhesion is easily liable to arise half-tone image parts formed with dots at the early stage of using a developer. With respect to carrier adhesion in solid image parts with the lapse of running time, it is problematic because a film-exfoliation of the used resin coating layer causes reduction in resistance associated with the reduced amount of the resin coating layer, and the reduced resistance causes carrier adhesion in solid image parts. Further, with respect to durability, there are problems with the amount of toner spent on the surface of carrier, destabilization of the charged amount associated with the toner spent, reduction in the amount of the coating layer caused by exfoliation of resin coating film, and reduction in resistance associated with the reduced amount of the coating layer. Therefore, with a carrier according to these related arts, there still remain problems that quality of copied images is degraded as the number of copied sheets increases, although excellent images can be formed in early stage of image formation.
In response to demands for being faster and more beautiful, high-quality of image and speeding-up are remarkably progressing in the area of image forming apparatus. In accordance with the movements, the amount of stress that a developer should go through has increased dramatically, and carriers that have been said to be long-lived are unable to have a sufficient operating life. In addition, conventionally, carbon black is frequently used as a resistance controlling agent of carrier, however, there is a fear of smearing a color image with carbon black, and such a smear is caused by exfoliated film of the used resin coating and/or desorption of carbon black. Therefore, various methods are proposed as a measure to counter the problem. For example, a carrier is proposed. The carrier is formed by making a conductive material (carbon black) reside on the surface of a core material, and the conductive material does not reside in a coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 07-140723). In addition, a carrier is proposed. The carrier has a concentration gradient of carbon black in the direction of the thickness of a coating layer, the concentration of the carbon black is gradually lowered toward the surface of the coating layer, and the carbon black does not reside on the surface of the coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 08-179570). A two-layered coating type carrier is proposed, the carrier comprises an inside coating layer and a surface coverage layer, the inside coating layer comprises a conductive carbon on surfaces of core material particles, and the surface coating layer comprises a white conductive material on the inside coating layer (see Japanese Patent Application Laid-Open (JP-A) No. 08-286429). However, these proposals cannot respond to the highly-stressful movements in recent years, and toner smears on images become more and more problematic, and production of carrier leaves room for improvement.
As a drastic measure against such smears on images, it is obvious that non-usage of carbon black which may cause smears on images is the most effective measure. However, when carbon black alone is not used in a carrier, as mentioned above, resistance of the carrier is increased, because carbon black is characterized in its low resistance. Generally, when a carrier having high resistance is used as a developer, with an image having a large surface area of copied image, the image density in the center portion is very low, only edge portions of the copied image are represented with thick density, and it is a so-called image that the edge effect is sharply represented. When an image is represented with letters and thin lines, a brilliant image is formed because of the edge effect, however, when an image is represented in neutral color tone, such a carrier has an disadvantage in that images having very poor reproductivity are formed.
For resistance controlling agents other than carbon black, for example, titanium oxides, zinc oxides or the like are known in the art, however, as to an effect of reducing the resistance, titanium oxides and zinc oxides do not have effect enough to replace carbon black. Accordingly, such resistance controlling agents do not lead to a solution of the problem, and there are still demands for further improvements and developments in carrier.